Equine/Animal Type Melanoma - Submitted on: Thursday April, 20, 2000

Contributed by:Artur Zembowicz, MD PhD.

Equine /Animal Type Melanoma (AMM) is a rare variant of malignant melanoma which is jet black and composed of a confluent proliferation of melanocytes with striking melanin synthesis reminiscent of vertical growth phase melanoma. It was first described in 1832 by William Dick, when he reported the existence of skin neoplasms in old gray horses, consisting of nodules of heavily pigmented cells and appearing after the geriatric transformation of their coat to white. Equine Melanotic disease, as it was termed then, affects primarily the perineum, vulva or penile sheath, the undersurface of the tail, the mammary glands, the inner surface of the lip and the external auditory meatus.   Though largely indolent, metastatic deposits may be found in the spleen, liver, bone marrow and parotid gland. By 1925, Darier recognized and described a similar lesion in humans, and ascribed to it the term "Melanosarcoma". Despite limited publications on this lesion in humans, AMM is considered a distinct clinical and pathological entity.   Consideration of the benign and malignant simulants will enable the reader to recognize the critical differentiating points.

Definition: Equine /Animal Type Melanoma (AMM) is a rare variant of malignant melanoma which is jet black and composed of a confluent proliferation of melanocytes with striking melanin synthesis reminiscent of vertical growth phase melanoma.

Clinical Features: AMM presents clinically as blue plaques or nodules averaging about 1 cm in diameter, but may be as large as 10 cm. There is no association with sun exposure, specific body site or family history of malignant melanoma or dysplastic nevus. Patients range in age from the first to the seventh decade, although the vast majority in most series occur within the first two decades of life. Among the 20,000 consultation and routine cases seen in the authors’ practice, during a six year period, six patients ranging in age from 9 to 85 years were diagnosed with this distinctive melanocytic tumor. The histology is characterized by striking melanin synthesis and an architectural growth pattern reminiscent of vertical growth phase melanoma by virtue of a confluent proliferation of melanocytes. The lesions were located on the back, scalp, ankle and sacrum, with 2 months to 18 years prior to clinical presentation. Clinically, all lesions elicited a putative impression of malignant melanoma. AMM has a potential for aggressive behavior that correlates with its cytomorphology. The presence of malignant nuclear features is associated with intracutaneous and regional lymph node metastases, generally less severe than nodular or superficially spreading vertical growth phase melanoma with similar histologic features.

Histology: Figures (Click on an image for a larger view) Figure 1. Animal Type Melanoma. The tumor is wedge shaped and extends to the subcutaneous fat. Note the absence of a grenz zone. Figure 2. Animal Type Melanoma. The tumor cells are filled with brown granular cytoplasmic pigment. Note the large, prominently staining nucleoli that distinguishes the neoplastic melanocytes from pigment laden macrophages. Figure 3. Animal Type Melanoma. Nodules are formed by the tumor cells. that are long and slender with coarse granules throughout. Figure 4. Animal Type Melanoma: Cells with dendritic processes are admixed with spindle and epithelioid cells, all heavily pigmented. Figure 5. Animal Type Melanoma: The tumor cells are intimately associated with sweat ducts and blood vessels. AMM shows prominent dermal involvement without a significant grenz zone of papillary dermal sparing and extension to the dermal-subcutaneous interface (Fig.1). Pigmentation of the tumor cells is heavy, ranging from dusty light brown deposits to dark coarse granules that obscure the nuclear cytology (Fig. 2). The latter pattern, when present in cells manifesting a polygonal or rounded morphology, obscures the distinction between the tumor cells and melanophages that are always present in the lesion in various numbers. These epithelioid tumor cells, unlike melanophages, often assume a nodular architectural disposition (Fig 3). Alternately, the tumor cells are sometimes spindled and disposed in fascicles. In either case, areas of confluent melanocytic growth are present, variably associated with necrosis. Typically, dissipation in cellularity occurs at the periphery of the lesion, where the cells manifest a dendritic appearance, morphologically reminiscent of a blue nevus (Fig 4). The neoplastic cells tend to infiltrate the adventitial dermis of follicular and adnexal structures (Fig 5.) This close association to the follicular sheaths, has prompted the application of the alternate appellation, pilar neurocristic harmartoma. Pagetoid infiltration of the epidermis is variable, mitotic activity is low and host response negligible. Overall, although cases can show atypical features, including necrosis, all criteria for malignant melanoma are not present.

Treatment: Despite the slow and benign course of most cases of AMM, the potential for aggressive behavior necessitates wide re-excision of all lesions, with 1 to 2 centimeter margins. Sentinel lymph node biopsies are also probably of value and should be performed in cases where this procedure is available. As in any invasive malignant melanoma, patients should be followed.

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